This invention relates to an acoustic transducer which not only reproduces a frequency range of interest, with high fidelity and improved efficiency, but which is also capable of reproducing rapidly varying frequency characteristics with high fidelity.
In the reproduction of tones and sounds it is, of course, necessary that the transducer be capable of reproducing a suitable range of frequencies, but applicant has found that in order to provide improved fidelity, it is also necessary to reproduce without any time lag frequencies which vary rapidly, such as, for example, those which occur at the start and at the end of a note, so that intonations, glissandos and fading notes can be faithfully reproduced, i.e., without any interfering frequencies arising out of the characteristics of the transducer. Until now, little attention has been paid to this problem, as conventional research, conditioned by the instruments used, was mainly restricted to the reproduction as a function of the frequency range. However, the human ear in particular is very sensitive to frequencies which vary over very short intervals and it is precisely such frequency changes that are characteristic features of certain musical instruments, particularly of string instruments and also of the players of such instruments.
Such phenomena which occur for very short time intervals are not well dealt with by conventional acoustic transducers or loudspeakers because the above-mentioned fine detail in lost in the reproduction due to the damping effects conventionally used in the prior art in an attempt to avoid undesirable resonances and distortion.
Acoustic transducers for converting mechanical or electrical energy into sound waves, and vice versa, necessarily include a coupling element or diaphragm which physically moves or is physically moved by the sound propagating medium. In the case of musical instruments, the coupling element may be a sounding board, for example, whereas in the case of loudspeakers, the coupling element is usually a thin light weight body in the form of a sheet or cone. In both cases, the coupling element or diaphragm will have inherent internal structural resonances, which, in the case of musical instruments may not be objectionable because such resonances add the color and quality of tone characteristics of that particular musical instrument.
However, in the case of loudspeakers for converting electrical energy into sound waves and vice versa, any inherent internal structural resonances in the diaphragm will cause distortions and reduce the fidelity of the transducing process. For example, if the diaphragm is made of thin material, the audio frequencies to be transduced will be propagated in the diaphragm itself, tending to cause the diaphragm to be momentarily corrugated. Such corrugations will produce increased rigidity in the diaphragm which will raise other existing resonances to a higher pitch, producing gliding tones which do not exist in the original sound to be transduced.
The above described phenomena is quite complicated and difficult to visualize. It is similar to the basic principles of operation of a well-known pseudo musical instrument commonly called a "singing saw" in which tones of different pitch are produced by varying the rigidity of the saw blade by simply bending it in differing amounts. However, the phenomena is much more complex and more closely resembles the Raman effect found in electromagnetic waves.
In the prior art there has been no attempt to reduce the "singing saw or acoustic Raman" effect because it has not previously been recognized.
Loudspeaker diaphragms are conventionally made of a fibrous material which frictionally resists internal bending forces. In addition, other damping elements are conventionally used which are either mechanically connected to the diaphragm or acoustically coupled to the second produced thereby.
However, damping devices also introduce distortion into the transducing process in addition to reducing the efficiency of the transducing process by absorbing energy therefrom. In other words, damping devices rely on frictional effects to avoid or reduce undesired resonances and such frictional effects are constantly changing in a dynamic system due to the great difference between the frictional forces present when two elements or fibers are at rest with respect to each other and the frictional forces present when they are in motion with respect to each other.
This effect is not limited to fibrous materials but is also present in other damping material such as plastics or rubber having high internal frictional losses. These materials have long chain molecules which take the place of fibers and rub on each other as they move with respect to each other in a dynamic system.
This effect is measurable and is responsible for certain colorations known to be present in all loudspeakers having paper or fibrous diaphragms.
In any event, both the "singing saw" effect and the internal friction effect, described above, reduce the fidelity of the acoustic transducing process.
It is a primary object of this invention to avoid the resonances in the frequency range of the transducers which produce the "singing saw" effect. By avoiding such resonances, the need for damping is eliminated.